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Chapter 12
12.1 The Causes of Weather Main Idea:
Air masses have different temperatures and amounts of moisture because of the uneven heating of Earth’s Surface.
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What is Meteorology? Meteorology is the study of atmospheric phenomena.
Greek: meteoros = high in the air
What is Meteorology? Anything that is high in the sky – raindrops, rainbows, dust, snowflakes, fog, and lightning – is an example of a “meteor”
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What is Meteorology? Atmospheric phenomena are classified into three major categories: Hydrometeors
Lithometeors
Electrometeors
Hydrometeors Cloud droplets & precip
Rain
Snow
Sleet
Hail
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Lithometers Smoke
Haze
Dust
Electrometeors Thunder
Lightning
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Weather Vs. Climate Weather:
Short-term variations in atmospheric phenomena that interact and affect the environment and life.
Weather Vs. Climate Climate:
Long term average of variations in weather for a particular area.
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Weather Vs. Climate An area’s climate (such as desert) is defined by meteorologists using over 30 years of weather data.
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Heating Earth’s Surface The crucial question for meteorology is how solar radiation is distributed around Earth.
Imbalanced Heating 3 Main Reasons
Daylight Hours vary on location
Angle of incoming sunlight varies on location.
Different surfaces absorb or reflect light differently.
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Thermal Energy Redistribution
The constant movement of air by temperature differences redistributes thermal energy around the world.
Weather is how Earth redistributes thermal energy.
Air Masses An air mass is a large volume of air that has the same characteristics, such as humidity and temperature, as its source region
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Air Masses Source region: The area over which the air mass forms.
Types of Air Masses There are FIVE types of air masses
that influence weather in the US.
Arctic (A)
Continental Polar (cP)
Continental Tropical (cT)
Maritime Polar (mP)
Maritime Tropical (mT)
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Arctic (A) Arctic air masses form over ice and
snow covered surfaces above 60N Lat. (Siberia and the Arctic Basin)
Brings bitter cold temperatures and dry air during the winter, and cold dry air during the summer.
Continental Polar (cP) Continental polar air masses
form over the interior of Canada and Alaska.
Brings frigid air southward in the winter, and cool dry air in the summer.
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Continental Tropical (cT) Continental tropical air masses
form over the southwestern US and Mexico.
Brings hot dry air to the US, especially in the summer.
Maritime Polar (mP) Maritime polar air masses form
over the cold waters of the N. Atlantic and N. Pacific oceans.
Brings cold-mild, humid air to the coasts. Source for heavy rain in the winter on the West Coast.
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Maritime Tropical (mT) Maritime tropical air masses
form over tropical bodies of water.
Brings hot, humid weather to the eastern two thirds of the US in the summer.
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Air Mass Modification Air masses move, transferring their
thermal energy and humidity from one area to another.
12.2 Weather Systems Main Idea: Weather results when air masses with different pressures and temperatures move, change, and collide.
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Global Wind Systems The directions of Earth’s winds
are influenced by Earth’s rotation.
The Coriolis effect results in fluids and objects moving in an apparent curved path rather than a straight line.
The Coriolis Effect
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The Coriolis Effect Moving air curves to the right in the northern hemisphere.
Moving air curves to the left in the southern hemisphere.
The Coriolis Effect The Coriolis effect and heat imbalances on Earth create distinct global wind systems.
They transport cold air towards the equator, and warm air to the poles.
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Global Wind Systems There are three basic zones, or wind systems, at Earth’s surface in each hemisphere.
The Polar Easterlies
The Prevailing Westerlies
The Trade Winds
Polar Easterlies Wind zones between 60N lat. and the north pole, and 60S lat. and the south pole are called the polar easterlies.
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Polar Easterlies Begin as dense polar air that
sinks.
As it sinks towards the equator, it is deflected in an easterly direction.
Polar easterlies are often weak and sporadic.
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Prevailing Westerlies Wind zones located between 30-
60N, and 30-60S are called the prevailing westerlies.
The prevailing westerlies are steady winds that move much of the weather across the US and Canada
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Trade Winds Between 30N-30S are two
circulation belts of winds known as the trade winds.
Air in these regions sinks, warms, and moves toward the equator in an easterly direction.
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Trade Winds When the air reaches the
equator, it rises and moves back towards 30 N and S lat., where it sinks and the process repeats.
Horse Latitudes The sinking air at 30 N and S lat.
From the trade winds creates an area of high pressure, associated with weak surface winds and Earth’s major deserts.
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Horse Latitudes
Intertropical Convergence Zone Trade winds from the North and
South converge at the equator, creating a low pressure region. (ITCZ)
Brings bands of cloudiness, and thunderstorms to the world’s tropical rain forests.
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Intertropical Convergence Zone
Jet Streams Jet streams are narrow bands of
fast wind that form at the intersection of wind zones.
Polar Jet Stream (major)
Subtropical Jet Stream (minor)
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Jet Streams Storms form along jet streams
and generate large-scale weather systems that follow the path of the jet stream.
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Fronts Air masses with different
characteristics can collide and result in dramatic weather changes.
A collision of two air masses forms a front.
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Cold Front Cold fronts occur when cold,
dense air displaces warm air. The cold air mass forces the warm air mass up sharply.
The results of a cold front are intense precipitation and thunderstorms.
Warm Front Advancing warm air displaces
cold air along a warm front.
It develops a gradual boundary slope.
The result of a warm front is widespread light precipitation
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Stationary Front When two air masses meet but neither
advances, the boundary between them stalls.
Frequently occurs between two modified air masses that have small temperature and pressure differences.
Light wind, mild precip.
Occluded Front A cold air mass moving so rapidly that it
overtakes a warm front, forcing the warm air upward and colliding with the cold air mass in front of the warm front is called an occluded front.
Strong winds and heavy precipitation are common along an occluded front.
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Pressure Systems
Pressure Systems Sinking air is associated with HIGH pressure (H)
Rising air is associated with LOW pressure (L)
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Pressure Systems Air always flows from areas of H pressure to areas of L pressure.
Air moves in a circular motion around H or L pressure systems.
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Low-pressure Systems In surface L systems, air rises.
Air outside the system spirals inward toward the center and then upward.
In the N. Hemisphere, air moves into L systems in a counterclockwise direction due to the Coriolis effect.
Low-pressure Systems
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Low-pressure Systems
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Low-pressure Systems L systems are often associated with cloudy weather and precipitation.
High-pressure Systems In surface H systems, air sinks.
Air moves away from the system’s center when it reaches the surface.
This air circulates outward in a clockwise direction due to the Coriolis effect.
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High-pressure Systems
High-pressure Systems H systems are usually associated
with fair weather.
They dominate most of Earth’s subtropical oceans and provide generally pleasant weather.
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12-3 Gathering Weather Data Thermometer
Barometer
Anemometer
Hygrometer
ASOS (Automated Surface Observing System)
Radiosonde and Rawinsonde
Radar (Radio Detection and Ranging)
Weather Satellites
12-4 Weather Analysis & Prediction
Vocabulary: Station model Isobar Isotherm Digital forecast Analog forecast
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12-4 Weather Analysis & Prediction
Several methods are used to develop short-term and long-term weather forecasts.
12-4 Weather Analysis & Prediction
Several methods are used to develop short-term and long-term weather forecasts.
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Station Models
After weather data are gathered, meteorologists plot the data on a map using station models for individual cities or towns.
Station Models
A station model is a record of weather data for a particular site and time.
Symbols are used to represent weather data.
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Station Models
Station Models
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Plotting Station Model Data Station models provide
information for individual sites.
To plot nationwide, meteorologists use lines that connect points of equal or constant values.
Plotting Station Model Data The values represent different
weather variables, such as pressure or temperature.
Isobars (equal pressure)
Isotherms (equal temperature)
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Isobars
Isotherms
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Interpreting Station Model Data
Inferences about weather can be made by studying isobars and isotherms on a map.
Interpreting Station Model Data
Isobars that are close together indicate a large pressure difference over a small area. (Strong Winds)
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Interpreting Station Model Data
Interpreting Station Model Data
Isobars that are far apart indicate small differences in pressure. (Light Winds)
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Interpreting Station Model Data
Isobars also indicate the locations of H and L pressure systems.
Frontal systems can be identified by combining isobar data with isotherm data.
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Types of Forecasts Meteorologists analyze data from different levels in the atmosphere, past and present, to produce a reliable forecast.
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Types of Forecasts 2 Main Types:
Digital Forecasts
Analog Forecasts
Digital Forecasts A digital forecast is created by applying physics and mathematics to atmospheric variables for computer analysis.
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Digital Forecasts Main method used by present
day meteorologists
Can be created quickly on a national or global scale.
Accuracy is directly related to the amount of available data.
Analog Forecasts Analog forecasts compare current
weather patterns to similar weather patterns from the past.
Difficult to find the same weather patterns from the past
Useful for conducting monthly or seasonal forecasts.
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Short-Term Forecasts The most accurate and detailed forecasts are short term because weather systems change directions, speeds, and intensities over time.
Short-Term Forecasts Extrapolation is reliable
Based on the data of surface and upper-level features, such as pressure systems.
Reliable for 1-3 day forecasting on temperature/precip
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Long-Term Forecasts Less reliable than short-term forecasts.
Based on circulation patterns in the atmosphere, and seasonal cycles.
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